In an important industrial process for producing pentaerythritol (PE), formaldehyde is reacted with acetaldehyde in the presence of metal hydroxide catalyst, and the main bulk of the pentaerythritol product is recovered from the aqueous reaction medium by crystallization. Typical pentaerythritol processes are described in U.S. Pat. Nos. 2,790,836 and 3,968,176.
The aqueous mother liquor resulting from the manufacture of pentaerythritol is a waste byproduct stream containing metal formate, unrecovered pentaerythritol, and soluble organic byproduct materials formed in the pentaerythritol synthesizing reaction, including lesser quantities of dipentaerythritol and tripentaerythritol, small quantities of pentaerythritol formals, and formose sugars which tend tocause some difficulty during crystallization of the pentaerythritol and continue to interfere with treatment of the aqueous waste stream.
This aqueous waste stream represents a difficult waste disposa problem, even though it contains metal formate (e.g., sodium formate) of substantial commercial value. If it is attempted to recover the metal formate by conventional evaporative crystallization procedures, these organic materials become concentrated in the crystallization mother liquor and cause it to become quite viscous, thereby hampering crystal separation from the liquor and contaminating the separated crystals. In practice, the recovery of useful metal formate from the liquor is limited to only about 75% of the total metal formate present, owing to these factors. Consequently, a substantial quantity of viscous organic-rich material is produced as a byproduct, which still poses a formidable waste disposal problem.
The potential for recovery of valuable byproducts contained in the aqueous waste stream of a pentaerythritol manufacturing operation has invited various developments which have been reported in the prior art literature.
U.S. Pat. No. 2,441,602 describes an aqueous waste stream treatment method which involves concentrating the stream by water removal, diluting the concentrated stream with a water-soluble monohydric alcohol and heating the diluted mixture, then separating the undissolved metal formate by filtration, distilling the filtrate solution to remove most of the water as an azeotrope with the alcohol, and subsequently cooling the solution to produce crystallized metal formate and pentaerythritol.
U.S. Pat. No. 2,617,791 describes a process for treating pentaerythritol mother liquor which involves heating the mother liquor with a fatty acid at 175.degree.-275.degree. C. until an oily phase and a solids phase are formed, and thereafter recovering the oily layer which contains fatty acid esters of the polyhydroxy compounds.
U.S. Pat. No. 2,780,655 describes a method of treating pentaerythritol mother liquor which involves concentrating the mother liquor to form a slurry of solid pentaerythritol and metal salts, adding formalin to the slurry to dissolve the pentaerythritol, and recovering the undissolved salts.
U.S. Pat. No. 3,179,704 describes a method of treating pentaerythritol mother liquor which involves evaporating the mother liquor to dryness, admixing the resulting dried solids with dimethylformamide to dissolve the organic materials, and separating the extract solution from the undissolved formate salts.
U.S. Pat. No. 3,766,277 describes a process which involves (1) contacting a pentaerythritol waste stream with tertiary-butanol to extract pentaerythritol; (2) optionally, purifying the extract phase with charcoal or macroreticular resin; and (3) recovering pentaerythritol from the purified extract phase by crystallization.
U.S. Pat. No. 3,875,248 describes a pentaerythritol recovery process which involves (1) passing pentaerythritol reaction product mixture through a macroreticular resin to remove organic materials, excepting pentaerythritol; and (2) subjecting the purified stream to concentrating and cooling procedures to precipitate crystalline pentaerythritol. The pentaerythritol product mixture is obtained directly from the formaldehyde/acetaldehyde reaction step and contains all of the pentaerythritol product. Preferably, the reaction product mixture is concentrated prior to the purification treatment with macroreticular resins in step(1).
Other United States Patents of general interest relating to the production and recovery of pentaerythritol and various byproducts include U.S. Pat. Nos. 2,004,010; 2,223,421; 2,270,839; 2,386,289; 2,696,507; 2,719,867; 2,782,918; 2,790,011; 2,790,836; 2,790,837; 2,820,066; 3,379,624; 3,478,115; 3,875,248; 3,968,176; 4,083,931; and references cited therein.
Because of environmental and economic considerations, as indicated by the prior art references disclosed hereinbefore, there have been continuing investigative efforts to develop methods for recovering the valuable organic and inorganic components of waste byproduct streams such as that associated with pentaerythritol production. However, the recovery processes that have been proposed for the pentaerythritol mother liquor have been hampered by difficulties with undesirably high viscosities, contamination by unwanted components, formation of new byproducts requiring additional treatment, and the like. Among such prior art recovery processes, for example, it is known to recover sodium formate from a pentaerythritol waste stream by evaporative crystallization and then to incinerate the aqueous mother liquor (containing, for example, 36-44% sodium formate, 14-18% pentaerythritol, and 18-22% of other organic impurities, in addition to water). However, the amount of inorganic material, in the form of suspended solids, in the waste stream being incinerated affects flame stability. Combustion of sodium formate causes a buildup of sodium carbonate that creates operational problems. Excessive amounts of water in the feed, in addition to other factors, results in low heat recovery which is economically undesirable.
There remains a need for a new and improved procedure for treatment of waste byproduct streams derived from large scale chemical manufacture.
Accordingly, it is a main object of the present invention to provide an improved process for recovering organic and inorganic values from the aqueous waste stream derived from pentaerythritol manutacture.
It is another ooject of this invention to provide a process for recovering as separate products of high purity substantially all of the metal formate and pentaerythritol contained in an aqueous waste stream derived from pentaerythritol production, in which production formaldehyde is reacted with acetaldehyde in the presence of a metal hydroxide catalyst.
It is a further object of this invention to isolate a residual organic byproduct concentrate of high fuel value from an aqueous waste stream derived from pentaerythritol production.
Other objects and advantages of the present invention shall become apparent from the accompanying description and examples.